A single 51.3 g stone exhibiting a chondritic texture and lacking fusion crust was found in the Sahara Desert. This meteorite became just another "typical" ordinary chondrite placed into a bag with many others that was purchased by Fabien Kuntz at the 2010 Munich Mineral Show. Upon closer examination, this small stone appeared to him to be an interesting unequilibrated stone, and on behalf of the Planetary Studies Foundation in Galena, Illinois, a sample was sent for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner). Northwest Africa 7135 was initially determined to be a unique ungrouped chondrite, potentially representing a new chondritic parent body (see the LPSC abstract).
Northwest Africa 7135 contains relatively small chondrules with olivine compositions that are more magnesian (Fa46) than chondrules in ordinary chondrites. At the same time, the chondrules are embedded in a metal-rich matrix that constitutes a larger volume of the meteorite (~30 vol%) than matrix in ordinary chondrites (~1015 vol%), but is similar to that in CO chondrites (Irving and Kuehner, 2015; Weisberg et al., 2006). Of significant importance is the presence of highly reduced sulfide phases in NWA 7135, including pyrrhotite, pentlandite, daubréelite, oldhamite and djerfisherite, along with the phosphide schreibersite; most of these reduced sulfides are unknown from other ordinary chondrites, but some are observed in E chondrites, winonaites, and in the unequilibrated forsteritic inclusions present in the Cumberland Falls aubrite. In a study of the inclusions in Cumberland Falls, Kuehner et al. (2016) ascertained that they are petrologic type 6, compared to the F chondrites Acfer 370, NWA 7135, and the most recently characterized El Médano 301, which are petrologic type 3/4.
Photo of a chondritic inclusion in the Cumberland Falls aubrite
National Museum of Natural History, Smithsonian Institution
Photo courtesy of Dr. Martin Horejsi
Oxygen-isotope values from six samples of NWA 7135 were obtained at the University of New Mexico (K. Ziegler), and the corresponding plots occupy a distinct space above the TFL on an oxygen three-isotope diagram (see below). These oxygen isotope values provide further evidence for a unique parent body unrelated to the known ordinary chondrites.
Diagram courtesy of the Meteoritical Bulletin: Oxygen Isotope Plots Direct Link
A forsterite (F) chondrite group was proposed in 1977 by Graham et al. to include the four meteorites Kakangari, Mount Morris, Pontlyfni, and Winona, along with certain inclusions in the Cumberland Falls aubrite. Subsequent chemical and O-isotope data from the Winona, Pontlyfni, and Mount Morris (Wisconsin) meteorites, and the Cumberland Falls inclusions, led to the establishment of the Winonaite (and 'W chondrite') group, with evidence that its members have a genetic relationship with the IAB complex irons (e.g., Davis et al., 1977). At the same time, continued research also determined that Kakangari represents the type specimen of a unique K chondrite grouplet. Still, the magnesian silicates in Northwest Africa 7135 are very similar to the xenolithic forsterite (Fa0.7) inclusions in Cumberland Falls (Kuehner et al., 2016), and it shares many characteristics with the anomalous chondrite Acfer 370 (see Moggi-Cecchi et al., #5421, 2009) and the ungrouped chondrite El Médano 301 (see Pourkhorsandi et al., #6176, 2016). In light of the current compositional and isotopic data, Kuehner et al. (2015, 2016) along with Pourkhorsandi et al. (2016) have recognized that NWA 7135, Acfer 370, El Médano 301, and the inclusions in Cumberland Falls (and in the ALHA78113 aubrite) represent a "new" F chondrite grouplet. They demonstrated that NWA 7135 and the other members of this F chondrite grouplet are clearly resolved from other ordinary chondrite reservoirs and form a cluster on an oxygen three-isotope diagram. Values for NWA 7135 and the other forsteritic meteorites and inclusions overlap and establish a unique trend line between the ordinary chondrites and the TFL (see diagrams below).
Diagram credit: Kuehner et al., 78th MetSoc, #5238 (2015)
Diagram credit: Kuehner et al., 47th LPSC, #2304 (2016)
An early solar system history for the F chondrite parent body was suggested by Neal and Lipschutz (1981). Their scenario involves formation of both the F chondrite and enstatite planetesimals in close proximity within the solar nebula. Thereafter, a severe collision between these two objects occurred, during which time F chondrite material became incorporated in the regolith of the enstatite planetesimal. This scenario is supported by their identification of shock-generated plagioclase and jadeitic pyroxene grains in the Cumberland Falls inclusions. In addition, Cumberland Falls exhibits shock effects from this collision, which include the presence of miniscule blebs of metallic FeNi and sulfide dispersed in the silicate grains producing silicate darkening, undulose to mosaic extinction with planar fractures in olivine, impact-melt clasts, and a shock stage of S2S3 (A. Rubin, 2010). Ultimately, this enstatite parent body experienced another impact event through which ejecta eventually became the Cumberland Falls aubrite. Other collisional ejecta eventually crossed paths with the Earth, which has been found and classified as E- and F-group chondrites and aubrite meteorites. Northwest Africa 7135 exhibits features of moderate weathering (W2) and very weak shock (S2).
Notably, a forsterite chondrite/achondrite clast with EH3 affinities has been discovered in the EH3 chondrite Sah 97158, paired with Sah 97096 (Boyet et al., 2011, #5120). The specimen of NWA 7135 shown above is a 0.58 g partial slice. The photos below show the complete meteorite as found and the cut face of the main mass, courtesy of Fabien Kuntz.